From DE 31 50 977 A1 methods and a device are known for determining and correcting guide defects of guided machine parts, the device corresponding to the device initially referred to. In a first method, relevant guide defects of a measuring machine are determined and approximated by correcting functions stored in the machine's computing unit. The incorrect results of the coordinate measuring device are then corrected in subsequent measurements by means of the correcting functions. In a second method, additional measuring devices are attached to the movable parts of the measuring machine to measure the distance to reference lines extending in the direction of displacement. With these measuring devices the guide defects are detected continuously during the measurement operation. The computing unit of the measuring machine uses the output signals of the additional measuring devices in order to correct the incorrect results of the coordinate measuring device. In a third method, the existing guide surfaces of the machine serve as reference lines, which is a less complex task, and the deviations once established by measurement against a linearity standard are stored in the form of an approximation function and used for permanent correction of the measurement values of the additional measuring devices. In this third method, which is a combination of the two methods first referred to, the tips of additional probes slide on the guide surfaces. To implement the two first-identified methods, use is made of a prismatic reference standard that extends parallel to the guide of a carriage. The reference standard carries on a first and on a second surface line gratings, one of which is two-dimensional. A position measuring system serves to determine the spatial position of a support member fixedly connected to the carriage, which support member has reading heads fitted to it and is guided on the reference standard. The reading heads or the support member are placed directly on surfaces of the reference standard. Such a measuring system which measures the displacement in a coordinate and detects at the same time the influence of any guide defects, is suitable, for example, for attachment to the guides of a measuring or processing machine with the aim to enhance its positioning accuracy considerably. In the first method it is assumed that the guide defects are constant and, once measured, can be stored as an approximation function to be considered in subsequent measurements. In the second method guide defects are measured continuously, yet only the relevant ones because of the increased complexity involved. The third method requires the use of an approximation function and the permanent correction of the measurement values of the additional measuring devices. In all the methods, the reference standard serves as a mechanical guide for the support member, which is always a disadvantage.
From DE 43 45 094 C2 a shape measuring machine is known, which includes a reference system for path correction on guideways that is decoupled from the machine's guiding means and control devices and comprises dimensionally accurate elongate standards disposed in parallel alignment to the individual coordinate axes of slidable carriages and adapted to be scanned continuously by parts movable on the guiding means. The reference system is not exposed to major forces and is constituted so as to have a geometry which is long-term stable with little sensitivity to temperature. While this known shape measuring machine distinguishes itself by displaying good guiding relationships for the slidable carriages, the complexity involved is considerable, in terms of both apparatus and measurement expenditure. The reference system is comprised of three standards, each formed by profiled rails of rectangular cross-section carrying scanning tracks on two adjacent lateral surfaces. The first standard is mounted on the machine bed. The second standard is attached to the end of a vertical column in addition to having at one end angle brackets embracing the scanning tracks of the first standard, so that the second standard is coupled to the first standard. Correspondingly, the third standard is coupled to the second standard. Provided at each of the two coupling locations between the three standards are several sensors which detect any relative movement between two standards at their coupling location in terms of their position and direction. The detected values are fed to a computing unit calculating therefrom corresponding correction values for the measurement result or for the spatial position of the probe element. This construction aims to enable the known shape measuring machine to exhibit an improved dynamic behavior and reduced non-productive times and durations of measurement as compared with a shape measuring machine with an extremely rigid machine bed, which is frequently manufactured from granite and supports, through a sliding carriage, the vertical column, which in turn is either a solid or heavy cast or steel welded construction.
From WO 99/17073 a device is known for the detection of six components of the relative motion of two bodies with one primarily two-dimensional translational basic motion. The bodies are formed of a plate provided with a two-dimensional optical grating and of an element having a plane surface parallel to the plate and including several optical gratings as reading head. The element is approximated to the plate in such fashion that the optical gratings enable detection of relative displacements, parallel to the plate, in the X and Y directions through evaluation of the intensity variations of the reflected light. To detect the distance between the element and the plate, provision is made for at least three distance sensing devices which are of the capacitive, inductive or probe type. To detect the spatial position of a carriage movable in a coordinate axis along a guide, that is, to detect a primarily one-dimensional movement, the known device is less suited because it is too complex for this purpose.
GB 2 034 880 A discloses an apparatus equally designed in particular to perform linear measurements in two dimensions. In addition, it provides a carriage movable along guide rails in two dimensions over a base. The carriage includes a surface upon which may be placed an object to be measured. The underside of the surface carries a first optical grating comprising parallel lines extending at right angles to one another. A probe is positioned above the surface. Secured to the base under the carriage are two reading heads for detecting movements of the carriage in two vertical directions. Due to its structural design this known apparatus is not suited either for the detection of the spatial position of a carriage movable in a coordinate axis along a guide of a machine tool or a measuring apparatus.
DE 43 08 456 C2 discloses a device which, for determining the position of a body to be positioned with respect to a prismatic reference body, detects the six degrees of freedom of the body to be positioned with respect to the reference body. The reference body carries on one of two surfaces disposed at right angles to one another two graduated scales, and on the other surface one graduated scale. The three graduated scales are designed as optical samplers in the form of CCD lines. The body to be positioned carries three projectors associated with the three graduated scales. Each projector emits three diverging light planes intersecting in a projection center, each light plane projecting three light marks onto the graduated scales. From the position of the individual light marks on the graduated scales it is possible to compute the spatial position of the projection centers longitudinally of the reference body and their distance to the corresponding surfaces of the reference body. These measurement values determine the position of the body to be positioned in its six degrees of freedom relative to the reference body.
It is an object of the present invention to improve a device of the type initially referred to so that defects of geometry in a linear axis of machine tools or measuring apparatus are without effect, which is accomplished by performing a continuous detection of a spatial position of the corresponding carriage directly, said device being desirably integrable into a machine or measuring apparatus construction.